An electrophotographic light-sensitive material may have various structures depending upon the characteristics required or an electrophotographic process to be employed.
Typical electrophotographic light-sensitive materials widely employed comprise a support having provided thereon at least one photoconductive layer and, if necessary, an insulating layer on the surface there-of. The electrophotographic light-sensitive material comprising a support and at least one photoconductive layer formed thereon is used for the image formation by an ordinary electrophotographic process including electrostatic charging, imagewise exposure, development, and, if desired, transfer.
Furthermore, a process using an electrophotographic light-sensitive material as an offset master plate precursor for direct plate making is widely practiced. In particular, a direct electrophotographic lithographic plate has recently become important as a system for printing in the order of from several hundreds to several thousands prints having a high image quality.
Under these circumstances, binder resins which are used for forming the photoconductive layer of an electrophotographic light-sensitive material are required to be excellent in the film-forming properties by themselves and the capability of dispersing photoconductive powder therein. Also, the photoconductive layer formed using the binder resin is required to have satisfactory adhesion to a base material or support. Further, the photoconductive layer formed by using the binder resin is required to have various excellent electrostatic characteristics such as high charging capacity, small dark decay, large light decay, and less fatigue due to prior light-exposure and also have an excellent image forming properties, and the photoconductive layer stably maintains these electrostatic properties in spite of the fluctuation in humidity at the time of image formation.
Further, extensive studies have been made for lithographic printing plate precursors using an electro photographic light-sensitive material, and for such a purpose, binder resins for a photoconductive layer which satisfy both the electrostatic characteristics as an electrophotographic light-sensitive material and printing properties as a printing plate precursor are required.
It has been found that the chemical structure of binder resin used in a photoconductive layer which contains at least an inorganic photoconductive substance, a spectral sensitizing dye and a binder resin has a great influence upon the electrostatic characteristics as well as smoothness of the photoconductive layer. Among the electrostatic characteristics, dark charge retention rate (D.R.R.) and photosensitivity are particularly affected.
Various investigations have been made on techniques for improvements in the smoothness and electrostatic characteristics of the photoconductive layer by using, as a binder resin, a resin having a relatively low molecular weight (i.e., a weight average molecular weight of from 10.sup.3 to 10.sup.4) and containing an acidic group. For instance, JP-A-63-217354 (the term "JP-A" as used herein means an "unexamined published Japanese Patent Application") discloses a resin having a polymer component containing an acidic group at random in the polymer main chain, U.S. Pat. No. 4,968,572 discloses a resin having an acidic group bonded at one terminal of the polymer main chain, U.S. Pat. Nos. 5,021,311 and 5,063,130, and EP-A-0389928 disclose a resin of graft type copolymer having an acidic group bonded at the terminal of the polymer main chain and a resin of graft type copolymer containing an acidic group in the graft portion, and EP-A-0432727 discloses an AB block copolymer containing an acidic group as a block.
It is presumed that these low molecular weight resins can act for sufficiently dispersing the photoconductive substance to restrain the occurrence of aggregation of photoconductive substance, and the acidic group thereof is sufficiently adsorbed on the stoichiometric defect of the inorganic photoconductive substance without hindering the adsorption of spectral sensitizing dye on the photoconductive substance and the resins mildly but sufficiently cover the surface of photoconductive substance. Also, it is presumed that even when the stoichiometric defect of the inorganic photoconductive substance varies to some extents, a relatively stable interaction between the inorganic photoconductive substance, spectral sensitizing dye and resin may be maintained since the resin has the sufficient adsorptive domain by the function and mechanism as described above. Of these resins, the graft type copolymer and AB block copolymer can provide a relatively stable performance even when ambient conditions are fluctuated.
Further, in order to obtain a satisfactorily high mechanical strength of the photoconductive layer which may be insufficient by only using the low molecular weight resin, various investigations have been made on techniques wherein a medium to high molecular weight resin is used together with the low molecular weight resin or wherein a resin containing a curable group is employed together with the low molecular weight resin and the layer containing these resins is cured after coating as described, for example, in U.S. Pat. Nos. 4,871,638, 63-220149, 63-220148, 4,968,572, 1-211766, 4,952,475, 5,084,367, 5,030,534, 5,009,975, 5,073,467, 5,077,166, 5,104,760, 5,104,759, 5,124,221, 3-92861, 3-92862, EP-A-0410324 and EP-A-0440226.
However, it has been found that, even in a case of using these various low molecular weight resins having an acidic group or in a case of using these low molecular weight resins together with medium to high molecular weight resins, it is yet insufficient to keep the stable performance in the case of greatly changing the ambient conditions from high-temperature and high-humidity to low-temperature and low-humidity. In particular, in a scanning exposure system using a semiconductor laser beam, the exposure time becomes longer and also there is a restriction on the exposure intensity as compared to a conventional overall simultaneous exposure system using a visible light, and hence a higher performance has been required for the electrostatic characteristics, in particular, the dark charge retention characteristics and photosensitivity.
Further, when the scanning exposure system using a semiconductor laser beam is applied to hitherto known light-sensitive materials for electrophotographic lithographic printing plate precursors, various problems may occur in that the difference between E.sub.1/2 and E.sub.1/10 is particularly large and the contrast of the duplicated image is decreased. Moreover, it is difficult to reduce the remaining potential after exposure, which results in severe fog formation in duplicated image, and when employed as offset masters, edge marks of originals pasted up appear on the prints, in addition to the insufficient electrostatic characteristics described above.
Moreover, it has been desired to develop a technique which can faithfully reproduce highly accurate images of continuous gradation as well as images composed of lines and dots using a liquid developer. However, the above-described known techniques are still insufficient to fulfill such a requirement. Specifically, in the known technique, the improved electrostatic characteristics which are achieved by means of the low molecular weight resin may be sometimes deteriorated by using it together with a medium to high molecular weight resin. In fact, it has been found that an electrophotographic light-sensitive material having a photoconductive layer wherein the above described known resins are used in combination may cause a problem on reproducibility of the above described highly accurate image (particularly, an image of continuous gradation) or on image forming performance in case of using a scanning exposure system with a laser beam of low power.